The blow molding of plastic containers is well known and practiced worldwide. Blow-molding offers many advantages over other forms of container manufacturing.
Among these advantages are: the ability to produce containers at minimal cost and with minimal waste; the low start-up costs and mold making lead times in comparison to other manufacturing methods such as injection molding; the ability to produce irregular shaped and hollow containers; the ability to produce containers quickly and automatically; the ability to produce containers from a variety of materials having qualities suited to the specific application.
Blow-molded containers having specially formed neck finishes are commonly employed for use with container closures. Neck finishes may be threaded for use with threaded closures, adapted for mating with snap-on closures, etc. It is common within a container blow-mold system to employ a main mold to form the container reservoir, and a neck block, or top block, to form the neck finish. Top blocks are also made to standard dimensions so that the parting face of the top block and main mold properly match. The main mold is usually adapted to interchangeably accept any standard top block for a particular container size or style. Top blocks and main molds are often aligned visually by being loosely engaged, tapped into alignment with a mallet, then firmly affixed together. The back side of each mold half, and the back side, or heel, of each top block, are affixed to a planar mounting or back plate. Such tapping and rigid engagement, usually by steel bolts driven into threaded holes in the main mold, may cause damage and wear to the mold system, particularly when performed repeatedly as is common. Provided that the depth of the main mold half, from its parting face to its back side, is exactly equal to the depth of the top block half, from its parting face to its heel, the mold system can be properly aligned and effective molding can be performed therein. It is important to maintain this alignment.
The first pair of components mounted to the top block are the neck finish blocks. Each neck finish block is usually made of aluminum, and may originally be formed as a circular body. The circular body is cut in half to form the neck finish block for each half of the mold. The neck finish blocks are machined to provide threads, ratchet teeth, shoulders or other surface features on the container which are used to engage or otherwise accommodate a closure.
During blow-molding, a parison of molten plastic is extruded between the open mold halves, then the mold system closes to entrap the parison within the mold cavity. A hollow blow pin is inserted through the neck opening, and into the parison where it inflates the parison with pressurized air to cause the parison to form to the shape of the container and neck finish cavity. The blow pin includes a hardened steel bushing, or shear bushing, having an annular blade, and each top block half includes a semi-circular hardened steel blade, or shear steel.
The shear steel shapes and trims flash from the top end of the bottle neck. By providing various degrees of precision ground undercut to the bottom surfaces of the shear steels, a distinct radius can be formed at the upper edge of the bottle neck area. This can improve the strength of the bottle neck in this critical area. Shear steels are generally made of precision ground and hardened steel.
When the mold is closed, the semi-circular blades and seals form annular orifices around the shear bushing. During molding, portions of the parison extending beyond the mold cavity are trapped between the paring faces of the mold halves and become unwanted flash. After the container is formed within the cavity and before the mold system reopens to release the blow-molded container, the shear bushing is retracted through the shear steel orifice. The shear bushing and shear steels are sized and shaped so that the retraction causes a shearing of the container opening through the neck, whose diameter is that of the shear bushing blade and shear steel orifice. After the mold container is removed form the mold, flash is removed by trimming in an automated process.
The last set of components which are mounted to the top block are the master seals, which are sometimes referred to as masters. These components provide a seal for the blow pin, thereby preventing air from escaping from the mold when the parison is inflated to form the container. They also form a “flash pocket” which pinches and seals the parison when the molds close at the start of the production cycle.
When all neck finish components, i.e. the neck finish block, the shear steel and the master seal, have been mounted to the top block halves, they are then typically secured thereto with four Allen cap screws. All neck finish components are aligned as closely as possible with the mold parting line. Failure to do so can prevent the mold from closing properly, leading to damage or even destruction of the neck finish components and/or blow pin.
Misalignment of the main mold and top block parting faces can result in other problems during attempted molding. In addition to damaging the neck finish component, unusual and hastened wear of the parting faces, excessive and untrimmable molding flash, concentrated and extreme pressures on areas of the mold and improper shearing of the container opening may occur.
The neck finish components are often the most detailed components of the mold system, having many features and requiring the highest degree of accuracy in manufacture. For instance, the mating halves of threaded neck finish blocks must match precisely and prevent mold flash to ensure that the container closure will properly fit onto the neck and seal the container opening. Interchangeable mold inserts are commonly employed within the top blocks to simplify and reduce the cost and time of making changes to the neck finish type. As a result, it is not so common to recondition or repair the top block itself as it is to recondition or repair a main mold which generally includes an integral cavity. This fact creates a problem and burden when main mold reconditioning or repair is required. Because the main mold depth will be reduced by such maintenance, the top block must also be reworked only to maintain an equal depth as the main mold, for proper alignment thereafter, or else a relief pocket must be precisely cut into the back plate to accommodate the new position of the top block's tail end. This is a costly and time consuming process.
Some of the disadvantages of conventional blow molding assemblies have been addressed. U.S. Pat. No. 5,776,518 discloses a top block assembly which allows alignment of the parting faces of the top block and/or neck finish components thereof with the parting face of the main mold. A method of providing such alignment is also disclosed. The disclosure of this patent is incorporated by reference herein.
The top block assembly according to the patent includes a top block including a neck finish portion and a rotatable cam. The cam bears against the forward face of a movable heel. It may alternatively bear against the neck finish components, namely the neck finish block, the shear steel and the master seal. The top block assembly is mountable to a main mold. By rotating the cam, a front surface of the top block can be aligned with a front surface of the main mold.
An assembly employing wedges to adjust the positions of neck finish inserts is disclosed in U.S. Pat. No. 5,894,024. The wedges are caused to move axially within guide slots by advancing or retracting screws.
U.S. Pat. No. 5,585,121 discloses a blow molding assembly that employs a screw having a cam head for positioning the shear steel and/or master seal. Movement of the shear steel towards the axis of the blow molding station is said to take up wear that has occurred in the shear steel.